The present invention relates to a device useful for separating or purifying chemical species in solution based on their relative rates of diffusion. More particularly this invention is directed to an apparatus and method for separation of solute species by their selectively enhanced diffusion through an oscillated liquid membrane.
The use of liquid membranes and the selected diffusion of substrates through liquid membranes have been the focus of significant research and development efforts in the art. Much work has been reported particularly with respect to the use of unsupported liquid membranes in the form of minute surfactant stabilized globules. Other liquid membrane structures known in the art are so-called supported liquid membranes wherein a liquid phase is contained within the confines of a porous support structure, for example by surface tension phenomena. The porous support structure is typically one which exhibits high affinity for the contained membrane-forming liquid. It is known also that the membrane-forming liquid can be modified to contain chemically reactive species or species which otherwise interact with solute species to promote or decrease the rate of diffusion of said species through the liquid membrane structure.
Industrial use of liquid membranes have been limited by problems associated with translating laboratory successes into industrial scale devices. Because of costs and difficulties associated with the creation and breaking down of stable emulsions, liquid surfactant membrane separation techniques provide an attractive alternative to traditional separation processes only when the target species must be extracted to very low concentration levels, for example, where metal ions or hazardous organic compounds are to be removed from water for discharge into the environment. Supported liquid membranes avoid many of the problems associated with emulsion technology, but they suffer from other practical problems such as limited surface area, solvent stability within the membrane structure and low solute species molecular diffusion rates.
The present invention makes use of supported liquid membrane structures in a device for purification or separation of selected solute species. However, the present device overcomes the problem associated with low diffusivity and concomitant poor membrane transport rates by oscillation of the supported liquid membrane. The present method and device embodying that method enables an enhanced mass transport of selected solute species across a supported liquid membrane. Applicant's discovery of oscillation enhanced membrane diffusion allows practical use of supported liquid membranes in separation/purification processes for a wide range of preparative and industrial applications.
Several investigators have published works dealing with the dynamics of single phase fluid flow through cylindrical tubes. Aris (Proc. Roy. Sec. A. 259, p. 370, 1960) discussed dispersion of a solute in pulsating fluid flow through a tube. Aris reported that the calculated diffusion constant for a viscous fluid can be proportional to the square of the amplitude of the pressure pulsations. For a viscous flow under a pulsating pressure gradient, the effective dispersion coefficient of a solute can be markedly enhanced. A similar theoretical analysis of this phenomenon has also been reported by Watson (J. Fluid. Mech.. 133, 233, 1983).
Experiments demonstrating an increase in molecular diffusivity in a gas filled cylindrical tube have been conducted by Joshi et al. (J. Fluid. Mech., 133, 245, 1983). Joshi found the increase in effective molecular diffusivity for axial transport of a contaminant gas subjected to oscillatory flow in a tube to be in agreement with the theoretical predictions of Watson. The experiment conducted by Joshi et al. measured methane transport through a gas filled tube. Experiments by Kurzweg and Howell (Phys. Fluids Vol. 27, No. 5, pp. 1046-48) have also indicated enhanced molecular gas dispersion coefficients for tubes of varying radii. Isotopic separation by differentially enhanced molecular diffusion of gaseous isotopes in a carrier gas has been reported by Howell (Phys. Fluids, 31 (6) 1988).